In the manufacture of thin film recording heads, the lithographic step that defines the write head is considered to be a critical one. The challenge of this process is to resolve a small physical dimension while maintaining tight width and shape control in a thick photoresist imaging layer. Specifically, said small dimension is the three dimensional patterning control of the isolated pole structure, namely the physical width and shape control over several microns in height, which poses the greatest challenge for this process. As the design rule target for the write magnetic track width continues to decrease, it becomes more and more important to maintain sufficient process margin, i.e. minimize CD (critical dimension) variation and maintain shape control through dose and defocus, in order to achieve the required pole geometries.
Several resolution enhancement techniques, such as phase shifting masks (PSM), optical proximity correction (OPC), and off-axis illumination have been shown to improve the process window for thin resist applications over that obtained using binary masks with conventional illumination techniques. The PSM methods can improve both the resolution and the process latitude for iso-(dense) line features imaged in thin photoresist. However, for thick resist applications, specifically iso-trench designs of the type with which the present invention is concerned, these benefits may be offset by unacceptable resist loss caused by the partial light transmission in the PSM mask design.
OPC masks are less expensive to manufacture than PSM masks, but the design and qualification of sub-resolution features can be problematic. Additional qualification iterations are not uncommon especially when changes in the device shape and/or size are required. Off-axis illumination typically works well for semi-dense to dense structures, but does not significantly increase the depth of focus for isolated features.
In this invention, a technique combining resolution enhancement techniques and off-axis illumination techniques has been shown to significantly improve the process metric for width and shape control. The technique is fully compatible with current manufacturing processes and can be implemented without generating additional integration issues.
FIG. 1 shows a typical design of a binary mask 12 for the write track width layer. The isolated clear area 11 seen in the figure defines the critical write track width. When light passes through the chrome free areas on the reticle, photochemical and thermal reactions in the photoresist enable the photoresist to be developed away, creating a mold-shaped isolated resist stencil. A metal film is then plated inside this mold to form the write pole. Manufacturing robustness for this process demands that the straight portion of the iso-trench resist profile be as high (or tall) as possible in order to accommodate many different plating thickness targets and alternate pattern transfer applications.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 6,749,972 (Yu) discloses an optical proximity correction method including adding scattering bars or anti-scattering bars and off-axis illumination. U.S. Pat. No. 6,807,662 (Toublan et al) describes adding an assist feature to enhance resolution in an isolated area. U.S. Patent Application 2004/0241557 (Ballman et al) teaches a mask having additional features. Off-axis illumination is mentioned.
U.S. Pat. No. 6,258,489 (Stanton et al) shows dummy features next to isolated or closely-packed features to equalize proximity effects. The resist is not fully etched through in the dummy features. In other words, since the dummy feature represents an opening in the resist, said dummy feature is not present in the developed pattern. Stanton et al. achieve this by giving the dummy feature the shape of a narrow frame. The outer edge of this frame continues to provide the out-of-phase diffraction fringes that are needed for OPC but the frame itself is too narrow to be resolved by the system so is absent from the photoresist image. Thus, this is the extension of the notion of scattering bars (which are single lines) from one dimension to two dimensions
As will be seen below, the present invention solves the problem in a different way.